June 14 (Reuters) - The number of Americans living with cancer will increase by nearly a third to almost 18 million by 2022, according to a report released on Th ursday by the American Cancer Society and the National Cancer Institute.

Researchers found that even though the incidence rates of cancer are decreasing, the number of cancer survivors is on the rise due to a growing - and aging - U.S. population. The incidence of cancer rises dramatically with age.

The report concluded that the expanding population of cancer survivors makes it increasingly important that the medical community understand their unique healthcare needs.

A survey presented last month showed that 94 percent of U.S. primary care doctors were unaware of the long-term side effects of some of the most commonly used chemotherapy drugs, highlighting the difficulties faced by patients after they beat the disease.

"Many survivors, even among those who are cancer free, must cope with the long-term effects of treatment, as well as psychological concerns such as fear of recurrence. As more people survive cancer, it is vital that health care providers are aware of the special needs of cancer patients and caregivers," said Elizabeth R. Ward, national vice president of Intramural Research and senior author of the latest report.

Nearly one-half of American cancer survivors are age 70 or older, while only 5 percent are 40 or younger. The median age of patients at the time of diagnosis is 66.

Almost two-thirds of survivors were diagnosed 5 or more years ago, while 15 percent were diagnosed 20 or more years ago, the report said.

The three most common cancers among men living with the disease are prostate cancer, colorectal cancer and melanoma, according to the report. The most common cancers among women living with cancer were breast cancer, uterine or colorectal cancers.

There are also 58,510 survivors of childhood cancer living in the United States, and 12,060 more children will be diagnosed in 2012. (Reporting by Debra Sherman; Editing by Michele Gershberg; Editing by Dan Grebler)